Thursday, April 20, 2006
A video image is comprised of scan lines, which are scanned quickly on the TV screen. The scanning of lines starts at the top of the screen and moves down to the bottom. The lines can be divided into two fields, which display the odd numbered lines and then the even numbered lines. This is called interlaced scanning or interlacing.
The lines of a video image can also be scanned progressively, which is what happens with digital televisions and computer screens. Rather than showing the lines in two different fields, with progressive scanning, the lines are displayed in a sequence, numerically.
Vertical scan lines in a video image have a direct relation to the quality of detail in that image. The more vertical lines in an image, the better the image. Today‚Äôs video systems (NTSC, PAL, and SECAM) have established numbers of vertical scan lines. NTSC has 525-line 60HZ for video display ‚Äì an interlaced display with two fields composed of 262 lines each. This is the standard for analog video in the United States. PAL uses 625-line 50HZ and is prevalent internationally in the use of TV broadcasts and video. This particular signal is also interlaced, with two fields made up of 312 lines apiece. This system produces a better image because it has more scan lines. SECAM is another 625-line 50 HZ system that‚Äôs used in parts of Europe and the Middle East.
Horizontal resolution differs depending on the quality of the TV screen. Horizontal resolution depends on the number of dots contained in each line on the screen from left to right. The average analog set can display 450 lines for horizontal resolution. Better quality sets can display higher numbers of horizontal lines.
With digital television (DTV), resolution is indicated by the number of pixels on the screen. Digital TV offers resolutions of 480 p, or 480 vertical lines progressively scanned; 720p, or 720 vertical lines progressively scanned, and 1080i, or 1080 lines scanned by interlacing. Digital TV offers a better image than analog TV due to the vertical scan. The overall detail is better, and this is part of the reason why digital TV is so popular. The more pixels that can be displayed on a TV screen, the better. Thus, the popularity of widescreen sets. Due to the overall superior image quality digital TV offers, it‚Äôs easy to see why DTV is the wave of the broadcasting future.